Short answer GoT had naked people. Plus it was the first medieval fantasy they ever saw in quite a while.
Tons of other medieval fantasy exist but most people didn't like watching anime or old movies.
Also people follow trends by years/generations. It is not until they grow up that they find out their trend was nothing new/different and it was probably dumb.
Remember how the squid games got super popular? But the concept wasn't new. It has been done in multiple anime. Remember among us? The concept isn't new it is the game of wink murder. Their cute animations plus streamers is what made among us boom.
But yeah, people love naked people on tv shows/series since its a good pretext to see naked people without getting judged. They also love beleiving that what they watch/play is unique and has never been done before.
Did I mention naked people? 🤣🤣🤣